Enable animated WebP by default.

The feature was LGTMed to ship here:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/yujPcy889e4

BUG=229641

Review URL: https://codereview.chromium.org/34083002

git-svn-id: svn://svn.chromium.org/blink/trunk@160162 bbb929c8-8fbe-4397-9dbb-9b2b20218538